1. Field of the Invention
The present invention relates to a pre-pit detecting apparatus for an optical recording medium having a recording surface with pre-pits carrying information concerning tracks repeatedly formed between the tracks.
2. Description of the Related Background Art
Recently, CD-R, CD-RW, DVD-R, DVD-RW, DVD-RAM and so on have been widely known as optical recording discs to which information data can be written. In addition, information recording/playing apparatuses to record and reproduce information by using such recording discs have been developed as products.
FIG. 1 is a schematic diagram of the area configuration of a DVD-RW as a recording disc described above.
As shown in FIG. 1, a DVD-RW disc has a data structure with a PCA (Power Calibration Area), an RMA (Recording Management Area), a lead-in area, data area, and a lead-out area from the inner circumference to the outer circumference of the disc. The PCA is an area for trial writing to determine the recording power of the laser beam and the RMA is an area in which management information related to recording is written. The lead-in area is provided with an embossed area formed as its part. The embossed area has phase pits formed on the disc in advance and sometimes records the information related to copy prevention.
FIG. 2 is a diagram of a part of the recording surface of such recordable disc.
As shown in FIG. 2, a disc substrate 101 has convex groove tracks 103 where information pits (marks) Pt to carry information data are to be formed spirally or concentrically, as well as concave land tracks 102 formed alternately on the board. Several LPPs (land pre-pits) 104 are formed between the adjacent groove tracks 103. The LPPs 104 are previously provided on the land tracks 102 to indicate recording timing and addresses of information data recorded by a disc recorder. The LPPs are formed as pits from the side of the disc substrate 101.
A disc player which plays an optical disc having such LPP's is provided with an LPP detection circuit. The LPP detection circuit is configured with a binarization circuit. Using its pickup, it receives the beam reflected from the optical disc with a light detector divided, for example, into two parts in the track tangent direction, and obtains the differential signal of the output signals from that optical detector, or the radial push-pull signal PP. The push-pull signal PP has a waveform as shown in FIG. 3 and the LPP component is a component projecting from the push-pull signal PP. Thus, by comparing the level of the push-pull signal PP and a threshold, a pre-pit detection signal PPD to indicate the detection of an LPP can be generated.
The pre-pit detection signal PPD has, as shown in FIG. 4, a level change like a pulse generated at every pickup reading position corresponding to an LPP. As shown in FIG. 4, a sync pulse PSYNC, which appears for every cycle T, is contained in the pre-pit detection signal PPD. The sync pulse PSYNC is followed by two pre-data pulses with a certain interval, but they do not always exist in every cycle to represent an address or other data. In FIG. 4, the third pulse from the sync pulse PSYNC is the pre-data pulse PD to carry the sector address. When recording information to an optical disc, the address on the optical disc is detected based on this pre-pit detection signal PPD for information recording.
However, even though the LPP and other pre-pits carry the addresses that are so important in information recording, the level of the section corresponding to the pre-pit in the push-pull signal is not constant. It is known to fluctuate depending on the pickup reading conditions such as tracking position. Therefore, it is difficult to set the threshold appropriately to binarize the push-pull signal.